問題描述

我遇到了有關 enable_if 和模板特化的適當用法的問題.

I am running into a problem regarding the appropriate usage of enable_if and template specialization.

修改示例后(出于保密原因)，這是一個可比較的示例:

After modifying the example (for confidentiality reasons), here's a comparable example:

我有一個名為less"的函數，用于檢查第一個參數是否小于第二個參數arg.假設我想要兩種不同的實現取決于輸入的類型 - 1 個整數和另一個雙倍.

I have function called "less" that checks if 1st arg is less than 2nd arg. Let's say I want to have 2 different kinds of implementations depending on the type of input - 1 implementation for integer and another for double.

到目前為止我的代碼看起來像這樣 -

The code that I have so far looks like this -

#include <type_traits>
#include <iostream>

template <class T,
          class = typename std::enable_if<std::is_floating_point<T>::value>::type>
     bool less(T a, T b) {
  // ....
}

template <class T,
          class = typename std::enable_if<std::is_integral<T>::value>::type>
     bool less(T a, T b) {
  // ....
}

int main() {
    float a;
    float b;
    less(a,b);
    return 0;
}

上面的代碼無法編譯，因為 - 它說我正在重新定義 less 方法.

The above code does not compile because - It says that I am re-defining the less method.

錯誤是:

Z.cpp:15:19: error: template parameter redefines default argument
          class = typename std::enable_if<std::is_integral<T>::value>::type>

                  ^
Z.cpp:9:19: note: previous default template argument defined here
          class = typename std::enable_if<std::is_floating_point<T>::value>::type>
                  ^

Z.cpp:16:11: error: redefinition of 'less'
     bool less(T a, T b) {
          ^

Z.cpp:10:11: note: previous definition is here
     bool less(T a, T b) {
          ^

Z.cpp:23:5: error: no matching function for call to 'less'
    less(a,b);
    ^~~~

Z.cpp:15:43: note: candidate template ignored: disabled by 'enable_if'
      [with T = float]
          class = typename std::enable_if<std::is_integral<T>::value>::type>
                                          ^
3 errors generated.

有人能指出這里的錯誤嗎?

Can someone point out what's the mistake here?

推薦答案

默認模板參數不是函數模板簽名的一部分.因此，在您的示例中，您有兩個相同的 less 重載，這是非法的.clang 抱怨默認參數的重新定義(根據 §14.1/12 [temp.param] 這也是非法的)，而 gcc 產生以下錯誤消息:

Default template arguments are not part of the signature of a function template. So in your example you have two identical overloads of less, which is illegal. clang complains about the redefinition of the default argument (which is also illegal according to §14.1/12 [temp.param]), while gcc produces the following error message:

錯誤:重新定義'templatebool less(T, T)'

要修復錯誤，請將 enable_if 表達式從默認參數移動到虛擬模板參數

To fix the error move the enable_if expression from default argument to a dummy template parameter

template <class T,
          typename std::enable_if<std::is_floating_point<T>::value, int>::type* = nullptr>
     bool less(T a, T b) {
  // ....
}

template <class T,
          typename std::enable_if<std::is_integral<T>::value, int>::type* = nullptr>
     bool less(T a, T b) {
  // ....
}

<小時>

另一種選擇是在返回類型中使用 enable_if，雖然我覺得這更難閱讀.

Another option is to use enable_if in the return type, though I feel this is harder to read.

template <class T>
      typename std::enable_if<std::is_floating_point<T>::value, bool>::type 
      less(T a, T b) {
  // ....
}

template <class T>
     typename std::enable_if<std::is_integral<T>::value, bool>::type 
     less(T a, T b) {
  // ....
}

這篇關于模板特化和 enable_if 問題的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網！